The invention relates to an imaging device incorporating a semiconductor wafer and more particularly to an improved silicon target support assembly for such a device.
Imaging devices such as silicon vidicons and silicon intensifier tubes employ sensing elements or targets consisting of single crystal silicon wafers. The operation of such silicon targets in these devices is well known in the art. Such an imaging device is described in U.S. Pat. No. 4,232,245 issued to E. D. Savoye et al., on Nov. 4, 1980 and incorporated by reference herein for the purpose of disclosure. The silicon target described in the Savoye et al. patent includes an input signal sensing surface and an oppositely disposed scan surface having a plurality of discrete PN junction storage diodes formed therein. It is common practice to thin the central portion of the input signal sensing surface of the target to a thickness of about 10 to 15 microns in order to obtain high resolution and to reduce lag. The thinned target is secured within the device either adjacent to the input faceplate in the case of a silicon vidicon, or in the target plane, spaced from the photocathode, in a silicon intensifier tube.
Applicant has discovered that thinning the central portion of the target causes the thinned membrane to wrinkle. In some targets, the wrinkles have a total excursion of about 0.25 mm which produce nonuniform variations in focus and shading in the output image. In applications where high resolution and good uniformity are required of the imaging device, it is necessary to minimize or eliminate target wrinkles.
U.S. Pat. No. 4,103,203 issued to S. A. Ochs on July 25, 1978 shows a thinned silicon target attached to an inner faceplate of a silicon vidicon by means of a transparent adhesive. While suitable for use in silicon vidicons where an input radiation species comprises photons, the transparent adhesive disclosed in the Ochs patent is unsuitable for use in silicon intensifier tubes of the type described in U.S. Pat. No. 3,761,762 issued to W. N. Henry et al., on Sept. 25, 1973 where photoelectrons from the photocathode of the intensifier portion of the device are incident on the silicon target. The adhesive and glass plate would prevent electrons from impinging on the target. A conventional target mounting or support structure adaptable for use in both silicon vidicons and silicon intensifier tubes is shown in U.S. Pat. No. 3,919,582 issued to S. A. Ochs on Nov. 11, 1975. This structure utilizes a formed support ring, a retainer ring and spring washer, all of equal diameters, to secure the silicon target near its periphery; however, the disclosed target mounting structure was proposed for a constant thickness silicon target without a thinned central portion. Such a mounting structure cannot remove target wrinkles.